1. Field of the Invention
The present invention relates generally to the field of electrohydraulic servovalves and servocontrol systems, and more particularly to an improved single-stage servovalve wherein a hard-over failure of one system component does not produce a corresponding failure of the hydraulically-controlled output.
2. Description of the Prior Art
Many aerospace vehicles utilize redundant control systems to improve reliability. In this context, redundancy means the presence of alternate control components, such that vehicle operation can continue if one component fails. The performance of the controlling component(s) is usually monitored. A hard-over failure of that component(s) is detected, and control is transferred to a standby component(s).
Servovalves are often used in such redundant control systems, and the valve's hydraulic output, which controls the load, is frequently used to assess system operation. Such monitoring often employs a differential pressure transducer or pressure switch communicating with the valve's output pressures, and a solenoid valve to disconnect that output from the load, and transfer control to another servovalve, should an excessive differential pressure be sensed.
In some redundant systems, it is desirable to nullify or neutralize the servovalve output immediately when a hard-over failure occurs. This avoids unnecessary transient behavior in the load being controlled, as might otherwise occur during the time interval between detection of the hard-over condition and the subsequent steps taken to remove it from the system. Such a fail-safe action has been achieved in a two-stage servovalve by adding additional spool lands and porting on the sliding spool, as shown in U.S. Pat. No. 3,922,955. The spool of this patent can operate in a region of normal four-way flow control on either side of the null position. When the spool in in a hard-over position abutting a stop, both control ports are closed and load motion will cease. Thus, a hard-over condition of the spool effectively terminates fluid control of the load.
However, when a single-stage servovalve is used in a control system, there is no second-stage spool to afford the fail-safe feature. Therefore, it would be desirable to provide an improved single-stage servovalve with a fail-safe capability.